<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input{
            color: #999;
        }
    </style>
</head>
<body>
    <input type="text" value="手机">
    <script>
        //1.获取元素
        var text = document.querySelector('input');
        //2.注册事件 获得焦点事件 onfocus
        text.onfocus = function(){
            // console.log('得到了焦点');
            if(text.value == '手机'){
                this.value = '';
            }
            //获得焦点需要把文本框里面的文字颜色变深
            this.style.color = '#333';
        }
        //2.注册事件 失去焦点事件 onblur
        text.onblur = function(){
            // console.log('失去了焦点');
            if(this.value === ''){
                this.value = '手机';
            }
            //失去焦点需要把文本框里面的文字颜色变浅
            this.style.color = '#999';
        }
    </script>
</body>
</html>